Out of cytogenetic studies of 4,400 consecutive newborn infants, 11 were found to have abnormalities of the sex chromosomes. None had distinguishing clinical abnormalities. These children, with ten normal controls, were evaluated at 1 and at 2½ years by child development specialists who were not informed as to which children had abnormal karyotypes. None of the children was mentally defective, but the control children showed a trend toward slightly higher developmental quotients than the study children. Of all sectors of development, language was the most affected, with language skills considerably delayed in the study children compared with the controls by the age of 2½. Further studies on these children are planned. Up to the age of at least 2½ years, abnormality of sex chromosomes was not found to cause mental retardation or specific personality deviations. It may be associated with increased vulnerability, with its greatest impact on the development of language. However, a range of developmental levels and personality has been demonstrated in these eleven children, and one cannot predict developmental potential from a knowledge of the genetic constitution.